396ci Small-Block - Tech

This popular big-block displacement comes in a new flavor!

Chevy stroker motors have been with us since almost day one but are hardly what you would call common. The advent, back in the ’80s, of cheap 400 cranks with reduced main bearings to fit a 350 changed the stroker scene forever. The 400’s 3.75-inch stroke gave a 0.030-over 350 some 383 cubes and, when done right, delivered satisfying results. So popular was this 350 stroker conversion that the supply of 400 donor cranks soon became an issue but Scat stepped in and saved the day by introducing a superior cast-steel crank at a cost less than a stock 400 crank. Building a stroker 350 was now so cost effective that the 383 became a major part of the Chevy small-block culture.

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IHRA Pro Stock racer Terry Walters of Terry Walters Precision Engines (TWPE) in Roanoke, Virginia, was one of those caught up in the 383 popularity wave but, unlike most, he got to build and dyno a lot of 383s.

As a result of his engine building skills, Walters’ Pro Stocker was one of the first into the 200-mph club. Such results on the asphalt dyno tell us in no uncertain terms that TWPE must be somewhere close to the top of the field when it comes to building power. In simple terms, winning performances in such a competitive arena are achieved by continually pushing the envelope. That same mind-set applies to the engines that Walters sells to his customers. For those who could afford the additional cost of a forged crank even more inches from a 350 were on the table, still at a cost-effective price. A 4.00-inch stroke in a 350 means that 408 inches are possible but Walters is not a fan of this combo. Here’s why and it’s proof that little in the way of optimal engine component combinations is simple.

Bigger Is Not Always Better

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If a 408 is possible, then why go for anything smaller? First, stuffing a 4.00-inch crank into a 350 block is way more problematic than say a 3.75-inch stroke as per a 383. Roughly speaking, four out of every 10 blocks fail the water jacket pressure test when the bottom of the bores and the pan rails are cut to clear a 4.00-inch stroke crank/rod combination. For a 383 only about one out of 10 fails the pressure test. This means anyone contemplating a 408 has to pay part of the cost of a 40 percent block scrap rate. This considerably ups the price for block prep of a 408.

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The next factors to consider are those of geometry. Cubes gained from a stroke increase have a small but, at the limit, significant negative in the way of piston friction. With a 4.00-inch stroke a 6-inch rod is about as long as is practical. It delivers a rod-to-stroke ratio of 1.5:1 and that is getting to be a little short. The short rod-to-stroke ratio means more piston side thrust and this, added to the fact that the crank is going to drag that piston farther up and down the bore, simply compounds the friction situation. The result is that although a 408 can deliver, getting the best from a 4.00-inch stroke is far more costly and the gains are disproportionately low to a combo with a little less stroke.

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So where do we stand if the stroke is limited to 3.875 instead of 4.00 inches? Here is Walters’ take on the situation. First, using a 3.875-inch stroke instead of 4.00 inches with the typical 0.030-inch overbore results in 12 less inches: 396 instead of 408. But this shorter stroke and a slightly better rod-to-stroke ratio cuts down on side thrust and means less piston-to-wall friction. The result is that we don’t lose the benefit of 12 cubes compared to the 408 but something more along the lines of 8 cubes. Also at this displacement, top end output is dictated more by the cylinder heads than the displacement. At the end of the day the 396 should make as good a top end as a 408 and have a torque output equivalent of about 8 cubes less than a 408. All this comes with greater reliability and a lot less cash investment in the block.

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For cranks and rods Walters works almost exclusively with Scat and not unexpectedly they have some major contributions toward the justification of the 396 as a top choice for a stroker 350 deal. Because of their extensive experience in the stroker business Scat continually strives to design cranks and rods that allow the most to be achieved within the confines of a 350 crankcase. Scat’s efforts in this department played right into Walters’ hands as far as being able to build a 396 for only the additional cost of a forged crank instead of the usual 383 cast-steel crank. Scat’s latest offerings allow a 3.875-inch stroke rotating assembly to just about fit where a typical 3.75-inch stroke normally fits. The bottom line here is that if Walters is right, we should get a 396 that delivers as good or maybe a shade better in terms of peak power while only sacrificing torque consistent with about 8, not 12, cubes less. All this sounds good in theory, so let’s see how it works out in practice.

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The Block

To build big power numbers, it requires a stout block, so the starting point for a TWPE 396 is eliminating the less-than-desirable candidates by means of sonic testing. After a block has been selected it goes through all the procedures one would expect of a top-dollar race engine. This includes cutting the clearances for the rods and the subsequent 100-psi pressure testing.

Rotating Assembly

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The heart of this 396 stroker build is the Scat 3.875-inch crank. There is considerably more to the design of a crank than you may at first suspect. For strokes as long as or longer than that of the 400 small-block (3.75-inch) an external balance is the norm because of a lack of crankcase space for the counterweights. For what it’s worth external balancing is a Band-Aid fix as it causes an increase in crank snout flexing at high rpm. This may not be an issue for a grannymobile but it can be for a high-rpm race unit. Most small-block Chevy cranks have insufficient counterbalance mass for the two inner cylinders. Being aware of this, Scat designed a crank that has the potential to be internally balanced to achieve the best possible solution. To make the counterbalance more effective, hollow rod journals are employed.

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Although designed to be internally balanced the Scat 396 crank still needs to be teamed with appropriate rods and pistons. First the pistons, not only must they be tough, have a top-notch ring package, but also be light to fall in line with an internal balance job. Here Walters uses pistons from SRP equipped with Total Seal rings. Why Total Seal? There are quality rings out there that cost less than the gapless Total Seal design but Walters’ answer here is: The dyno tells me I should use them!

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Now we come to the rods. This is a critical aspect of the build. The problem with many rods is that they require a lot of block cutting to give the bolt shoulder clearance with both the block and the camshaft. In addition to this, the bolt head also runs into the pan rail so that also needs to be cut for clearance. The Scat stroker rod aggressively addresses these issues. By utilizing top-grade material for the rods, minimizing bolt thread engagement to the shortest length possible without compromising strength and a compact design on the cap’s bolt platform Scat has reduced the rod’s mass in the areas most critical to a stroker. The result is that the rod actually has as much as 0.060-inch clearance with a stock base circle cam.

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Now you may ask what’s the deal here? It takes next to nothing in the way of effort to order a reduced base circle cam. As Walters will tell you in short order a lot of effort went into the utilization of as big a base circle cam as possible in his Pro Stockers. The reason is that the bigger base circle cams allow for more lifter acceleration before roller lifter side thrust limitations are reached. Also, preserving as large a base circle as possible allows for a more aggressive profile (a key element in power production) while still retaining streetable reliability. So, in a secondary way, the Scat rod combined with the 3.875-inch stroke is worth power because it allows a more aggressive cam to be used. The cam is a subject discussed in more detail below, but for now it should be clear that in addition to costing a lot less than a 408 stroker this 396 is looking good in the way of output potential.

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Heads

Compared to a stock mill, this engine has a lot of cubes to fill. Without exceptionally good heads those cubes will be starved of air and be underutilized. Here, Walters favors heads from AFR and Dart. For this build the heads are Dart 200cc runner castings hand-ported by a highly experienced and championship-winning head porter friend of Walters’. The intake runners started at an average of 197 cc and, when finished, at 208 cc so not much came out of them. The key here was flow from an efficient port and not a big port, that way the port velocity was higher. This has the benefit of delivering better low-speed output as well as enhancing the top end. Also, a high-efficiency intake valve seat was used that delivers low and mid range flow figures more akin to a 2.150-inch intake valve rather than the 2.08-inch used. Conceptually, the exhaust ports were the same as the intakes in as much as they were modified for maximum flow from a highly efficient shape rather than just a big port. Velocity probing showed that the velocity distribution across the port was far more even than most heads we have seen. This is yet another factor toward good low-speed manners. In short, these heads had more than enough flow to easily exceed 600 hp from a race 350 so it would be interesting to see what our 396cam’d more for the street and with just a 10.5:1 CRwould do.

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Valvetrain

There are four factors that dictate whether or not a valvetrain can be considered successful. These are reliability, valve event timing, valve lift, and, of course, cost. The valvetrain for our 396 was a very conscientiously spec’d setup centered around what, until recently, COMP Cams called their Busch profiles. These profiles are fairly aggressive to make really good power yet, when accompanied by appropriate valvetrain parts, pack the reliability to be streetable. Part of this reliability equation is the fact that the Scat rod will allow the use of a stock base circle cam. This not only cuts block-wearing side-loads but also improves dynamics. As for valve events, be aware that what works optimally for a 350 won’t even be within 25 hp of what’s needed for this 396. In this instance the cam profile used was a COMP 4875S on both intake and exhaust. These lobes have an advertised duration of 280/242 degrees at 0.050. The lobes are on a 106 LCA and, because of the strong low- and mid-lift flow of the Dart heads, the cam was timed in at 1 degree retarded instead of the more normal 4 degrees advanced. Just for the record, there is about 12 hp and an equal number of lb-ft difference between these two timing points.

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The COMP Cams Busch grind has a 0.381 lobe lift and to get the valve lift needed for outputs of 580 hp plus a 1.75:1 rocker ratio is needed. For the build here, a Crane set was used because Walters’ client for this engine already had a set with only an hour or two of use. The final valvetrain ingredient, and it’s of major importance, are the valvesprings. These are made by PAC for a third party, which is where Walters sources them. With only moderate spring forces on the seat and over the nose (net lift is 0.650) this spring will allow the valvetrain to turn a perfectly behaved 8,150 rpm with steel valves. This means it has plenty of margin for the 7,500 redline as our 396.

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Induction & Ignition

Just like everything else, there is nothing inconsequential about the induction system. The carb, which flows some 960 cfm, is specifically (available through TWPE) built for Walters by AED in Richmond. At 960 cfm it may be big but it services the engine as if it were a 750 down low and a 960 up topjust what is needed for a street/strip engine. The AED carb is mounted on a mildly prepped Edelbrock Super Victor, and for spark a 9,000-rpm capable Performance Distributors HEI with a custom curve was used.

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Dyno Time

The engine was loaded on the dyno and broken in. Next, it was given a post break-in service and made ready for some serious pulls. Nothing was very far off optimal on the first pull but at TWPE they don’t use the dyno to see how much horsepower the engine hasthey use it to see how much more they can find. After optimizing cam timing, fuel, and ignition this 396 pumped out 603 hp and 538 lb-ft of torqueall on pump gas.

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Now those are impressive numbers but let’s put that into prospective. First, a stroker is supposed to be about making torque. Sure, any stroker will make more torquethe question here really is did it make as much as it should have? A good torque output per cube for a streetable 10.5:1 small-block Chevy is 1.3 to 1.32 lb-ft per cube. That means to qualify as good our 396 needed to make 515 to 523 lb-ft. It made 538 lb-ft so on that basis it scored very well. As for horsepower, anything that is truly a street driver that makes 1.5 hp or better per cube can be rated as excellent. Our 396 made 1.522 hp per cube. Within the same cost, compression ratio, and cam duration bracket most 408s don’t crack the 1.5 hp per cube or 600 hp.

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For a build like this from TWPE figure $8,500 as entry level and about $9,800 for a replica of what’s presented here. (TWPE can also supply parts and subassemblies at very competitive prices.) So the price is good and that leaves us with only one question: What’s not to like? CHP